An optical isolator for coupling light from a first waveguide to a second waveguide is disclosed. The optical isolator utilizes a resonator coupled to the first and second optical waveguides. The resonator has a resonance at .lamda. for light traveling from the first optical waveguide to the second optical waveguide; however, the resonator does not have a resonance at .lamda. for light traveling from the second waveguide to the first waveguide. The resonator can use a layer of ferromagnetic material in an applied magnetic field. The magnetic field within the ferromagnetic material varies in strength and/or direction over the layer of ferromagnetic material. The magnetic field can be generated by an external magnetic field that varies over the layer of ferromagnetic material. Alternatively, the resonator can include a layer of ferromagnetic metal that overlies a portion of the layer of ferromagnetic material and a constant external magnetic field.